This invention relates to components and methods of making them. It has particular application to a sub-miniature fuse for electronic components and most particularly for surface mount devices where small size, low energy actuation, low resistance, high frequency signal handling, and high open resistance are desired. As used herein, the term "sub-miniature" indicates a component less than 0.1" on a side in at least two dimensions. The invention will be described in connection with such fuses, but the utility of some aspects of the invention is not limited thereto.
In some of its aspects, the present invention is a modification of the structures and processes described in commonly owned U.S. Pat. No. 4,749,980, the disclosure of which is hereby incorporated by reference.
With the advent of surface mount technology, burning and charring of surface mount boards by runaway components has become much more prevalent. The closer proximity of components, as found on surface mount boards, contributes to this problem along with thinner dielectric materials required to reduce component size. In addition, the area available to conduct away or radiate energy during normal operation or catastrophic failure is reduced.
Large, high component density, surface mount boards may cost thousands of dollars in today's market so that the protection offered by fused components can result in an extreme cost savings over the life of the board or the equipment incorporating such a board. The complete destruction by fire of the equipment or structure in which these components are housed is also prevented by proper fusing at the surface mount board level.
Surface mount monolithic ceramic capacitors, electrolytic (e.g., tantalum) capacitors and power transistors are typical of some of the components that can produce board burning and charring during failure.
A fuse to protect these and similar components from generating destructive temperatures on surface mount boards must be small enough to be incorporated within the housing of the component or externally attachable to the housing so that no additional board real estate or change in component footprint is required.
The fuse must have extreme reliability to be effective and must not be subject to loss in reliability due to complicated and variable manufacturing procedures.
Such a fuse must have the lowest possible impedance, even when operating at high frequencies of 100 MHz or more, so that losses in the fused component are reduced to an absolute minimum.
The fuse must carry a significant current without serious overall increases in impedance to the series-connected component, yet open rapidly with a small increase in current before the component approaches its critical failure temperature. For example, one specification for a fuse for a tantalum capacitor requires that the fuse carry 0.75 amperes D.C. for five seconds but must blow within five seconds on application of 1.4 amperes D.C.
The open fuse must have a very high resistance so that minute residual currents can not flow through the protected component over long periods of time. In the case of tantalum capacitors even the continuous flow of a few microamps can reestablish high temperatures in the failed component, so that a resistance on the order of ten megohms may be required in the open fuse.
Finally, the fuse must be able to be manufactured economically and reliably using high volume techniques such as those found in the semiconductor industry.
U.S. Pat. Nos. 4,107,759 (Shirn et al.), 4,107,762 (Shirn et al.), and 4,193,106 (Coleman) are among the earlier patents that discuss the problems of fuse protection for capacitors. These patents use exothermic wire fuses buried in molded plastic housings in thermal contact with the capacitor. They have proven to be an unreliable solution because of serious thermal variables that can prevent actual exothermic action due to chilling of the wire link. If the exothermic wire does not ignite, the fuse may carry enough current to ignite the tantalum capacitor.
U.S. Pat. No. 4,224,656 (DeMatos et al.) is similar to the foregoing patents, but shows a method for isolating the exothermic wire in space to overcome the erratic behavior of exothermic wire molded in plastic.
U.S. Patent No. 4,814,946 (Su) discloses that exothermic wire is used for protecting capacitors because the reliability of low melting temperature metals as a fusible link in a capacitor assembly is very poor. Su therefore uses a bimetallic exothermic wire, made of aluminum wire, with a ruthenium or palladium cladding, and covered with a silicone adhesive composition. This wire ignites at a temperature of around 650.degree. C. and reaches a maximum temperature during its reaction of about 3000.degree. C.
All of these patents suffer from high manufacturing costs due to difficulties in handling tiny wire, high impedance at high frequencies, and difficulties with termination of the wire to the outside of the package.
The necessary small diameter fuse wire, on the order of one mil, is extremely hard to fabricate into a surface mount package and causes relatively high manufacturing cost because manufacture is not tractable to mass production methods such as found in the semiconductor industry.
The small surface area of small diameter wires impedes high frequency signals which flow only on the surface of a conductor, thereby increasing the high frequency impedance of the fused component. In addition, small diameter wires show significant inductance. The effective series resistance (ESR) of the fuse is therefore generally objectionably high when used in high frequency applications.
The extreme small diameter of the exothermic wire is necessary to bring a short length of it to the exothermic reaction temperature and requires that the fuse have a relatively high D.C. resistance, thereby adding to the overall impedance of the fuse component combination. I have found that making the link element flat, or placing it in contact with a heat sink, prevents reproducible ignition of the fuse link under the desired overcurrent conditions.
U.S. Pat. No. 4,757,423 (Franklin) forms a fused tantalum capacitor in another way. This patent utilizes as the fuse link, a pad of spherical polystyrene particles coated with about 1% by weight of a metal and molded at high temperature and pressure into plaques, in which the metallic shell continuity is preserved in a continuous polystyrene matrix formed from the coated particles during the molding operation. This approach eliminates the tiny wire problem in a tantalum capacitor fuse, but it introduces new variables that are difficult to control. The overall D.C. resistance and current carrying characteristics of the fuse are so sensitive to the polymer and metal phase ratio in the matrix along with the need for precise control of internal and external geometries that a practical fuse to protect a tantalum capacitor becomes extremely difficult to manufacture. Moreover, the polystyrene particles are easily damaged at temperatures encountered in surface-mount techniques.
U.S. Pat. No. 4,749,980 (Morrill et al.) discloses a fuse whose link has a large surface area, hence a low D.C. resistance and ESR, but the fuse shows too high a residual resistance for use in an electrolytic capacitor and is difficult to make small instance, a standard "D" sized capacitor package.